In chemical processing plants, such as oil refineries, extensive use is made of heat exchangers. In some heat exchangers, the tubes are arranged horizontally, often at some height above the ground, and in other cases they are arranged vertically.
In use, over time, a deposit tends to build up on the inner surfaces of the tubes, the nature of the deposit depending on the fluid flowing through the tubes. The build up of deposit increases flow resistance and reduces the thermal conductivity of the walls of the tube, both effects reducing the efficiency of the heat exchanger.
When efficiency drops to an unacceptable level, the interior of the tubes of the core has to be cleaned. This is usually carried out by high pressure jetting. After access is gained to the ends of the tubes, for example by removing a header or an access plate in a header, a lance emitting jets of water at high pressure is inserted into the tube and advanced along its length. The process is repeated for each of the tubes until the entire core is cleaned.
Often, it suffices to use a flexible lance that can simply be advanced into the tube by hand. However, for harder deposits, it is sometimes necessary to use a rigid lance and to rotate the lance as it is being advanced into each tube. As the lance may be several meters long, special apparatus is needed to support and advance the lance.
Existing apparatus used for this purpose comprises a rigid beam resembling an RSJ (roller steel joist). A carriage slidable along the top of the beam comprises a motor and a gearbox for rotating the lance as well as a rotary coupling for feeding water under high pressure into the lance while allowing it to rotate. The lance itself passes through a hole in each of a series of guide plate that are themselves slidable along the top of the beam and spaced apart by springs. These plates are needed to stop the lance from bowing as it rotates.
The major problem presented by existing apparatus for supporting and advancing a rotating rigid lance is one of weight. The apparatus is too heavy to be raised to the height of the tube to be cleaned without the use of a hoist or a crane. Furthermore, in order to position the entire apparatus in correct alignment with the tube to be cleaned, it is necessary to construct scaffolding. The weight of the apparatus also adds to the cost of its transportation.